The present invention relates to blood filters and particularly to continuous, high-volume blood filtering devices suitable for handling blood during heart surgery, kidney dialysis, etc.
In blood transfusions and heart surgery, it has become routine to employ blood bank blood. This is prepared by withdrawing blood from donors, adding heparin or other anti-coagulants, and then storing the blood under carefully controlled conditions until its use is required.
Whether it is used directly in transfusions or in heart surgery, there sometimes have been observed in the patient an ensuing cessation of circulation and other reactions not attributable to mismatching of blood types. Although the cause of this regrettable result is not known, it is recognized as a function of blood storage duration. Accordingly, it is common practice to discard blood bank blood after it has been stored for a predetermined time.
Another difficult problem exists when handling blood during heart surgery, kidney dialysis, etc. In these procedures not only is the blood altered to produce harmful constituents, but it becomes contaminated with extraneous material such as epithelium, pieces of muscle, fat emboli, fragments of suture material and entrained air. This foreign material must be removed in order successfully to consummate the surgical procedure.
In my patent application entitled "Blood Treating and Filtering Apparatus", Ser. No. 882,663, filed Dec. 5, 1969, now U.S. Pat. No. 3,593,854, I describe a blood filtering unit for removing blood debris, harmful constituents, and entrained air from blood at a high-volume rate suitable for use in open heart surgery. This apparatus has proved effective and reliable in the surgical setting for which it was designed. However, continued surgical experience with the unit has shown that the fibrous filter mat used in the unit tends to become clogged at its upstream, or presenting, surface before the filtering capacity of the filter pad is reached, thereby shortening the life of the filtering unit, and adding expense and inconvenience to the blood filtering operation.
The causes of filter surface clogging can be appreciated from the following brief description of the surgical blood filtering unit disclosed in the above-cited U.S. Pat. No. 3,593,854. This filtering unit comprises a flow chamber and a fibrous filter mat laterally supported within the chamber by upstream and downstream perforated discs. In the assembled unit, the perforated discs are coextensive with, and firmly abut, the upstream and downstream surfaces of the filter mat, exposing at the disc perforations a plurality of small flat surface regions having uniform fiber density. In particular, the fiber density at the presenting surface regions is essentially the same as the fiber density throughout the filter mat. Because the filter mat density must be sufficient to filter small cellular debris in the blood, the presenting surface regions of the filter mat must be necessarily filter such small debris also. As a result, blood debris tends to stack upon the filter pad surface, and this, coupled with the limited area of the presenting surface regions, leads to premature clogging.
The present invention overcomes the above-described problem of clogging at the filtering surface. In the apparatus of my new design, the filter mat is supported upstream by a grill having a plurality of substantially deep ribs for indenting the upstream surface of the mat, thereby forming a plurality of fluffed surface regions, each of graded fiber density, in the interrib spaces. The fluffed surface regions serve both to increase the presenting area of the filter mat and, more importantly, to permit size-selection filtration of blood debris at the filter pad surface. Experimentally, the filtering unit of the new design has a capacity for blood cell aggregates approximately two to three times that of the old design filtering unit described in U.S. Pat. No. 3,593,854.
It is therefore a specific object of the present invention to provide a blood filtering apparatus which overcomes the problem of surface clogging associated with the above-described old design filtering apparatus, and thus to provide a high capacity blood filter suitable for use in open heart surgery.